This invention relates to thermionic cathodes and especially to an improved dispenser-type thermionic cathode.
Dispenser types of thermionic cathodes are usually fabricated by forming a porous metal matrix. The pores of the matrix are filled with an active cathode material, e.g., compounds of alkaline earth metals (Ba, Ca, and Sr), or a reservoir of these compounds is formed behind the matrix. During operation of the cathode, activating materials, e.g., Ba and BaO, are generated within the pores or are supplied from a reservoir behind the matrix. The activating materials must then migrate through the pores, either by surface diffusion or gaseous flow, and be supplied to the emitting surface in sufficient quantities to maintain good surface coverage so as to insure adequate electron emission from the surface.
These matrices are usually formed by compacting a quantity of metal powders of approximately 8 micron size and then sintering the mass at high temperature. (The temperature range depends on the type of metal powder used.) The pressure and temperature are adjusted to give a matrix having the desired pore volume and size. The pore structure (size, total volume, and interconnections) is critical to the proper operation the cathode and the pores in these matrices are very convoluted. If the pores near the back of the matrix (i.e., the side opposite the emitting surface) do not form an interconnecting path with other pores which open upon the emitting surface, cathode life will be shortened since material in those pores cannot reach the surface. An inherently bad feature of such a structure is that, since the connections between pores are randomly generated, the path length that the active materials must traverse to reach the emitting surface can be many times the actual matrix thickness (typical thicknesses are about 1-4 mm). It has been demonstrated that these factors limit the life and emission of conventional dispenser cathodes. Also, since these matrices cannot be examined nondestructively to determine if their pore structure is satisfactory, bad cathodes which will fail prematurely are sometime installed in tubes.